1. Field of the Invention
This invention relates, generally, to solid state load protection systems and more particularly to ground fault sensing enabling the system to detect a ground fault which is in excess of a predetermined value for a predetermined period of time and to thereby cause a trip signal to be generated so as to deenergize a load.
2. Description of the Prior Art
It is known that some load protection systems have various protection functions such as current overload, phase loss, and ground fault sensing. These functions generally provide a trip time for a circuit breaker and associated load which is inversely related to the square of the current flowing in the load to be protected. Therefore, an inverse time relationship is generally utilized whereby the time between a fault which is sensed and trip out are inversely related such that a high overcurrent will result in a very short period of time for device trip out and a relatively low overcurrent fault will result in a substantially longer time before trip out. Types of devices which accomplish the above can be found in U.S. Pat. No. 4,021,703, "Phase Imbalance Detection Circuit", issued May 3, 1977 to Gary, et al; U.S. Pat. No. 3,996,499, "Zener Diode Effect on Long Acceleration Module", issued Dec. 7, 1976 to Gary, et al.; U.S. Pat. No. 3,818,275, "Circuit Interrupter Including Improved Trip Circuit Using Current Transformers", issued June 18, 1974 to Shimp and U.S. Pat. No. 3,602,783, "Circuit Breaker Device Including Improved Overcurrent Protective Device", issued Aug. 31, 1971 to Engle, et al., all assigned to the assignee of the present invention. However, the above-mentioned patents do not generally provide ground fault sensing as part of a protective system or require the use of an additional current transformer.
It is desirable to have a ground fault sensing system which the user may use with a load protection system and is more simple to use. It is also desirable to have a ground fault sensing system which does not require the substantial addition of components or which must be permanently wired into a load protection system or that require the partial or complete dismantling of a load protection system.